1. Field of the Invention
The present invention relates to a belt-and-pulley type continuously variable transmission, and more particularly to a belt-and-pulley type continuously variable transmission having a driver pulley on an input shaft, a driven pulley on an output shaft, each of the driver and driven pulleys comprising fixed and movable conical members, and a V-belt trained around the driver and driven pulleys, the movable conical members of the driver and driven pulleys being supplied with oil under pressure from a servo hydraulic pressure chamber for controlling the effective diameters of the pulleys thereby to continuously vary the transmission ratio between the input and output shafts, i.e., the driver and driven pulleys.
2. Description of the Relevant Art
Belt-and-pulley type continuously variable transmissions for use on motor vehicles are required to transmit the torque effectively by tensioning a V-belt trained around driver and driven pulleys to a degree proportional to the torque to be transmitted as by applying side fluid pressure to the V-belt. The V-belt should remain tensioned when the effective diameters of the driver and driven pulleys are to be varied in order to vary the transmission ratio.
Japanese Laid-Open Patent Publications Nos. 55-65755 (published May 17, 1980) and 60-125450 published July 4, 1985) disclose V-belt-and-pulley-type continuously variable transmissions in which the pressure of oil from a pump is regulated according to the rotational speed of a driver pulley (i.e., the rotational speed of an engine) and a transmission ratio, and the regulated oil pressure is supplied through a slide valve that is controlled by the rotational speed of the driver pulley and the opening of a throttle valve to a servo hydraulic pressure chamber which applies hydraulic pressure to axially move a movable conical member of the driver pulley in its axial direction, so that the effective diameter of the driver pulley can be controlled while keeping a side pressure imposed on the V-belt.
In the above conventional transmission, however, the effective area in the servo hydraulic pressure chamber for actuating the movable conical member of a driven pulley is smaller than that in the servo hydraulic pressure chamber for actuating the movable conical member of the driver pulley, and the hydraulic pressure chamber associated with the driven pulley is always supplied with regulated oil pressure. When the movable conical member of the driver pulley slides for varying the transmission ratio for a higher vehicle speed, the effective diameter of the driver pulley is increased so that the tension of the V-belt is increased to move the movable conical member of the driven pulley against the side pressure thereon for thereby reducing the effective diameter of the driven pulley. Therefore, the load on the V-belt is increased, affecting the durability of the V-belt. Conversely, in order to slide the movable conical member of the drive pulley in the direction of a larger speed ratio, the oil pressure in its servo hydraulic pressure chamber is released to decrease the tension of the V-belt so that the side force on the driven pulley is utilized to move its movable conical member against the tension of the V-belt for thereby increasing the effective diameter of the driven pulley. In this mode of operation, the side pressure on the driver pulley may unnecessarily be reduced.
U.S. Pat. No 3,600,961 issued Aug. 24, 1971 discloses a control structure utilizing higher-pressure oil of a constant pressure and regulated lower-pressure oil which can be regulated by a transmission ratio or the like, the lower-pressure being applied to impose a side pressure on driver and driven pulleys. When the transmission ratio is to be changed, the higher-pressure oil is selectively supplied to a servo hydraulic pressure chamber associated with the pulley which is of a greater effective diameter. The disclosed arrangement is also disadvantageous in that the load on a V-belt is increased to the extent which affects the durability thereof. In some transmission ratio range, a higher side pressure than the necessary side pressure has to be applied, resulting in a large pressure loss.
The two Japanese Laid-Open Patent Publications, referred to above, are also problematic in that when the servo hydraulic pressure chamber of the driver pulley is connected to a drain passage to lower the pressure in the chamber thereby to increase the transmission ratio, most of the oil is drained from the servo hydraulic pressure chamber and air is allowed to enter the servo hydraulic pressure chamber, whereupon the driver pulley fails to apply the desired side pressure on the belt. Moreover, when regulated oil pressure is thereafter introduced into the servo hydraulic pressure chamber of the drive pulley, air remaining therein is first compressed, and then the transmission ratio starts to be changed. As a result, the transmission does not operate with a good response, when the transmission ratio thereof is to be again decreased.
Japanese Laid-Open Patent Publication No. 60-159456 published Aug. 20, 1985 discloses a belt-and-pulley type continuously variable transmission having driver and driven pulleys associated with servo hydraulic pressure chambers which have different pressure-acting areas, respectively, the hydraulic pressure chamber of the driven pulley being supplied with regulated oil pressure at all times. When the hydraulic pressure in the hydraulic pressure chamber of the driver pulley is low, low-pressure oil is supplied thereinto to prevent oil from being drained. Therefore, the disclosed transmission is free from one of the drawbacks as described above. However, the transmission shown in this publication suffers from the following shortcomings:
1. Where the transmission is employed in a motor vehicle, mutually related signal oil pressures representing the opening of the throttle valve and the rotational speed of the engine act on the opposite ends of a slide valve, and the fixed port of a servo hydraulic pressure chamber is connected selectively to an oil pressure passage and an oil drain passage for varying the transmission ratio. In such an arrangement, however, there is a speed range in which an upshift and/or downshift cannot be performed, resulting in that. Accordingly, the transmission ratio cannot freely be controlled in some speed ranges.
2. The speed at which the transmission ratio varies tends to be constant at all times.
The above problems 1 and 2 can be solved by a belt-and-pulley type continuously variable transmission disclosed in Japanese Laid-Open Patent Publication No. 60-136655 published July 20, 1985. In this disclosed transmission, a regulated oil pressure is supplied to the hydraulic pressure chamber of a driver pulley through a directional control valve and a speed control valve which are disposed hydraulically in series with each other and controlled by solenoid-operated valves, respectively. The directional control valve disposed upstream of the speed control valve serves to change the direction in which the transmission ratio varies, and the downstream speed control valve serves to control the speed at which the transmission ratio varies. According to the disclosed transmission, the effective area of the movable conical member of a driven pulley is smaller than that of the movable conical member of the driver pulley, and the hydraulic pressure chamber of the driven pulley is supplied with the regulated oil pressure at all times. Therefore, the problem that the load acting on the V-belt increases when varying the transmission ratio cannot be solved by this transmission.
The present invention has been made in order to effectively solve the problems of the conventional belt-and-pulley continuously variable transmissions for use in motor vehicles.